Have you tried uninstalling your antivirus instead of Thunderbird?

Thunderbird does not actually differentiate file types. Attachments go through a conversion process when attached converting them to mime encoded text. So in an email, they are all the same (text).

If you are having issues with a specific file type as you appear to be, then the immediate response is to look outside of Thunderbird for something else on your system that has an interest in files, and files of a particular type. The most common and obvious here would be the antivirus. PDF files are a known vector for malware, so most antivirus programs do different things with PDF to other known vector files as they area formatted binary file. So the antivirus is the most logical starting point. It might just be the antivirus is struggling with the file system being slow and it is therefore still scanning when the email goes out.

In a general housekeeping sense, Window does a rather poor job of managing orphaned temp files, so your temp folder might be chock-a-block full of leftover garbage and be the root cause of the A/V struggling. Almost as long as there has been windows, users have complained it gets slower he longer you have it, with some pundits suggesting regular installed of Windows to keep it humming along. One of the things these refreshes do (other than force a lot of software to be reinstalled and in Thunderbird case mail lost) is clear out the temp folder. In this instance, Google threw up a link to Trend Micro for instructions on cleaning up the temp folder. I have linked to them, fundamentally as I don't want to write out the instructions and secondly to offer another point of view as to why you should do this housekeeping. I do not use, not recommend Trend, they just appeared in my first page of search results and their information was factual as far as it went.

It is possible that some other PDF file manager software you have (like adobe) could be causing issues but it is far less likely that an antivirus issue.

Thanks Mat for such quick response,

it's Windows 10 Security and no other A/V

There was MalwareBytes, that I've uninstalled

I've disabled it and also turned off Firewall

Still, no go.

Regards Zarko